24 ECHINODERMATA. 



tentacula, in a quiescent state, are retracted within the body, the skin 

 of the upper part closing over them. Thus the animal may then appear 

 entirely divested of external organs. When it is active, and the suckers 

 in powerful adhesion, the tentacula, rising from the disc by a stout cylin- 

 drical stem, are beautifully displayed in circular arrangement, each dis- 

 charging its external functions separately. These exercises, as we shall 

 see of others, are also most interesting to the spectator. 



There is much difference in the quality of the skin, as above alluded 

 to. That investing the body, from the shoulder upwards, bears the 

 strongest resemblance to the appearance of birch bark. The surface of 

 the disc and of the tentacula, are of this pecuhar description, which adds 

 to the beauty and singulaiity of the parts. The tentacula, whose actual 

 structure is she^vn in Plate II. figs. 3, 4, are luxuriantly arborescent. 

 The hollow cylindrical stem is half an inch in diameter, and all the parts 

 divergmg from it in circular arrangement. 



Each tentaculum sweeps through the water, and then curves into the 

 central orifice of the disc, in a contracted state, as explained more mi- 

 nutely of the suljjects illustratmg the following paragraph. 



The distension of these organs is obviously from the absorption of 

 water, though by what channels is unknown. On this point naturalists 

 are not agreed. There can be no doubt, however, of copious absorption, 

 which Ls proved both by the great dimensions of the body, and by the 

 jet from the posterior extremity jjroducmg a vortex at the surface, 

 while the orifice is far below. The jet is so vigorous as sometimes to 

 spout eight or ten inches, and over the side of the vessel. 



The colour of the HolotJmria pentades is various. On the whole 

 brown predominates, lighter or darker, and diverging from different 

 shades into mulberry or purple. Large specimens are dark as chocolate : 

 some are wood-brown. Two small specimens, which I concluded the 

 pentades, with a double row of suckers in the stripes, were grey. About 

 a fifth part of the circumference of the body is frequently much paler 

 than the rest, which is particularly conspicuous in large specimens ; and 

 thence do some naturalists conjecture this the beUy of the animal. One 

 such portion, comprehending two stripes of suckers, was yellowish-brown. 



